Using a genetic murine model for opioid addiction and linkage analysis, we identified a chromosome 10 locus (later defined as the mu oploid receptor [OPRM1] gene) that determines 50% of the variance in how much morphine a mouse consumes during a two-bottle choice paradigm experiment. This study is consonant with phenotypic evaluations of OPRM1 gene knock-out mice, which are indifferent to the rewarding properties of morphine. These murine studies suggest that variation in the OPRM1 gene may convey some risk for opioid dependence (OD). Twin and family studies of OD are consistent with a heritable component of risk that is attributable specifically to opioids. This application proposes to study OD patients and matched controls, to examine OPRMI gene sequence variants as potential susceptibility factors for OD. We have determined, through multiplex sequencing, 25 common transcribed / regulatory OPRM gene variants. Preliminary analysis suggests that some OPRM promotor variants (which may alter transcription factor binding sites) may be found at higher frequency among OD persons compared to controls. These preliminary results indicate the need for a more extensive examination of the OPRM1 gene sequence in a genomically-controlled association study of sufficient power to detect a small effect. In addition, variation in the pro-opiomelanocortin cDNA and other opioid cDNAs will be assessed in case-control association studies of OD probands and ethnically-matched controls, using haplotype analysis. These studies may identify genes of the endogenous opioid systems as risk factors for genetic susceptibility to OD.